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4d47's solution

to Nth Prime in the PHP Track

Published at Jul 13 2018 · 1 comment
Test suite

Given a number n, determine what the nth prime is.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

Running the tests

  1. Go to the root of your PHP exercise directory, which is <EXERCISM_WORKSPACE>/php. To find the Exercism workspace run

     % exercism debug | grep Workspace
  2. Get PHPUnit if you don't have it already.

     % wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
     % chmod +x phpunit.phar
  3. Execute the tests:

     % ./phpunit.phar nth-prime/nth-prime_test.php


A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.



require 'nth-prime.php';

class NthPrimeTest extends PHPUnit\Framework\TestCase
    public function testFirstPrime()
        $this->assertEquals(2, prime(1));
    public function testSecondPrime()
        $this->assertEquals(3, prime(2));
    public function testSixthPrime()
        $this->assertEquals(13, prime(6));
    public function testBigPrime()
        $this->assertEquals(104743, prime(10001));
    public function testZeroPrime()
        $this->assertEquals(false, prime(0));

function prime(int $n): int {
    if ($n <= 0) {
        return false;

    for ($i = 2, $count = 0; $count < $n; $i++) {
        if (gmp_prob_prime($i)) {
    return $i-1;

Community comments

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Avatar of 4d47

Okay I know I'm cheating with gmp ;-) This elaborated answer is great read.

What can you learn from this solution?

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